Field of the Invention
The present invention generally relates to a robotic gripper for automated pick-and-place operation having extensive applications in micro-manipulation and micro-assembly tasks. More specifically, the present invention relates to a compliant gripper having a larger grasping range and having an integrated dual-sensitivity, dual-range force sensor for the detection of both grasping and interaction forces and having an integrated position sensor for detecting the position of an actuation tip during micro-assembly.
Related Art
Various sensorized grippers with different structures, actuators, and sensors have been proposed for micro-handling applications. For instance, an electrostatically actuated micro-gripper with capacitive type of position and grasping force sensors has been reported. Piezoelectric cantilever-based grippers with tip position and grasping force sensing have been well studied and compliant grippers driven by piezoelectric stack actuators and sensed by strain gauges have been developed. Also, compliant grippers with self-sensing capability have been recently developed.
Compliant mechanisms deliver attractive merits in terms of no clearance, no friction, no wear, no need for lubrication, and ease of manufacture. Hence, they have been widely employed in micro-handling applications.
Depending on targeted applications, compliant grippers can be fabricated in macro-, meso-, and micro-scales. Both linear and rotary guiding approaches have been proposed in gripper structure design. For example, a macro-scale flexure-based gripper with linear guiding and double displacement amplifications has been proposed which can provide a maximum gripping range of 134 μm. A meso-scale buckling compliant gripper using simple rotary guiding has been devised to achieve a wide gripping range and a narrow range of force variation for small-scale application. In addition, a micro-scale MEMS gripper with rotary guiding has been developed which delivers a gripping range of 94 μm, and more recently, a MEMS rotary gripper with both arms actuated by rotary comb-drives and sensed by electro-thermal sensors was reported as providing a gripping range of 90 μm.
However, a majority of the existing compliant grippers can only deliver a limited range of gripping (less than 1 mm, typically).
To adapt to the gripping of objects of various sizes, it is desirable to design a gripper with large gripping range so as to facilitate a wider application. However, it has been challenging to create a gripper with a large gripping range while maintaining the overall dimensions of the gripper as compact as possible.
In addition to the above deficiency, gripper arms are generally only able to provide a unidirectional gripping motion. For example, in a normally open gripper, the gripper arms are usually actuated to close the gripper tips (in positive direction) while the open operation (in negative direction) is realized by the restoring force provided by flexural mechanisms. However, in practice, the unidirectional gripping range is limited by the allowable maximum stress of the material. Hence, the capability of drive in both positive and negative directions allows the generation of a double-stroke over the unidirectional drive for the same mechanical design.
For instance, if a bidirectional actuation flexure gripper designed to provide an operation range of D holds the same operation range D using a unidirectional actuation, the flexures should be designed to have longer lengths which lead to a larger dimension of the gripper mechanism. Thus, the bidirectional actuation is necessary to generate a contact structure design for the gripper. However, few grippers have been reported to provide the drive in both close and open operation of the gripper tips. One reason is that majority of actuators can only provide a one-way drive, e.g., the aforementioned electrostatic actuators, piezoelectric stack actuators, etc. To implement the drive in both positive and negative directions, two actuators can be adopted. However, this complicates the gripper design and increases the hardware costs.
Moreover, to realize an accurate and reliable grasp operation, position and force sensing are crucial. When the gripper arms are closed to grasp an object, the gripping force sensing is important to guarantee that an appropriate force is exerted on the object. The reason lies in that a small force is not sufficient to grasp the object firmly while a large force may incur damage to the object. In addition, to ensure a reliable operation, it is also essential to detect the interaction force exerted by environment. Detection of an interaction force is important to determine whether the gripper contacts the environment and to ensure the safety of the gripper device by avoiding excessive interaction. Most existing grippers can only sense gripping force in the gripping direction whereas few grippers can sense interaction forces in other directions.
In related art, computer vision has been employed to detect contact force. However, the limited view of a microscope restricts its wide application. To overcome this problem, a thermally actuated gripper with capacitive type of grasping and interaction force sensors has been proposed where two force sensors are used to detect the grasping force and interaction force along two perpendicular directions, respectively. In addition, a miniature gripper with a piezoresistive sensor for the measurement of contact position and magnitude of a perpendicular external interaction force has been developed. However, these grippers require separate force sensors for sensing the grasping and interaction forces. This complicates the design of the gripper structure and increases hardware costs. The reason for using two force sensors mainly arises from the fact that, generally, a force sensor can only measure force in a single direction. In view of the forgoing, it has been challenging to devise a force sensor for the detection of the applied forces in two or three perpendicular directions.
In addition, the sensitivity and measurement range of a sensor often conflict with one another: improving sensitivity often decreases the measurement range while increasing the measurement range often decreases sensitivity. Thus, it has been difficult to develop a force sensor exhibiting high sensitivity while simultaneously having a large measurement range.